The present invention relates to a method of fabricating a nitride crystal of a Group III element, such as GaN, AlN, InN, or the like, a mixture, a liquid phase growth method, a nitride crystal, nitride crystal powders, and a vapor phase growth method.
A method of easily fabricating a Group III nitride bulk crystal of GaN, AlN, InN or the like has never existed. If a GaN bulk crystal can be made, its impact is beyond imagination for realization of a blue laser diode and the like. A bulk crystal of a wide band gap semiconductor material will be fully used in the 21st century.
As a method of fabricating GaN crystal powders, a method using the reaction of a Ga oxide, such as Ga.sub.2 O.sub.3, with ammonia has been put into practical use. Powders fabricated by the method are available on the market as a reagent.
A method of easily fabricating a Group III nitride bulk crystal has not been developed. Although the following literature by S. Porowski et al. is published, since it calls for a very high pressure, it is dangerous and difficult to fabricate the crystal using this method. In addition, the size of the obtained crystal is as small as about a few mm.
"Prospects for high-pressure crystal growth of III-V nitrides" by S. Porowski, J. Jun, P. Perlin, I. Grzegory, H. Teisseyre and T. Suski, Inst. Phys. Conf. Ser. No. 137: Chapter 4.
Paper presented at the 5th SiC and Related Materials Conf., Washington, D.C., 1993.
A Group III nitride represented by GaN has a very high melting point and is decomposed and sublimated at a temperature lower than the melting point. Consequently, its melt cannot be produced and a crystal cannot be grown from the melt. Since the solubility of the Group III nitride to produce a Group III solution is very low, it is also difficult to grow a crystal from the solution, only three methods of growing a crystal of a Group III nitride have been put into practical use, and they are vapor phase epitaxial growth methods of HVPE (Hydride Vapor Phase Epitaxial growth), MOVPE (Metal Organic Vapor Phase Epitaxial growth), and MBE (Molecular Beam Epitaxial growth). LEDs made of a GaN manufactured by using such methods are available on the market. As an example of the GaN crystal growth by MOVPE, the following literature was published.
"Novel metal organic chemical vapor deposition system for GaN growth", S. Nakamura, Y. Harada and M. Seno, Appl. Phys. Lett. 58(18)6, 1991.
In recent years, attention has been given to the use of a nitride crystal of a Group III element represented by GaN as a material for a blue light emitting device. In order to produce the device, for example, it is necessary to epitaxially grow a GaN crystal on a substrate. In the epitaxial growth, it is ideal when the lattice constant and the coefficient of thermal expansion of the crystal as a substrate and those of a crystal which is grown on the substrate are the same in order to prevent occurrence of distortion in the crystal to be grown. However, a bulk crystal of a nitride which can be used as a substrate has not been obtained. A sapphire substrate or the like having a different lattice constant has to be used instead, and then a crystal is epitaxially grown on the sapphire substrate.
Although the MOVPE method is mainly used as an epitaxial growth method at present, there are problems in that an organic metal as such a material catches fire easily and is dangerous, and is expensive, and needs a large-scaled complicated expensive growth system. Hydrogen is inevitably contained as an impurity in the grown crystal and it is consequently difficult to increase the concentration of carriers of a p-type crystal.
In order to increase the carrier concentration of the p-type crystal, it is therefore necessary to arrange a p-type dielectric layer on the surface of a device and to perform a nitride crystal process. Since it is necessary to arrange the p-type dielectric layer on the surface of a device, the degree of freedom in designing the device structure is low.
Another epitaxial growth method is an LPE (Liquid Phase Epitaxial growth) method in which a Group III nitride crystal is dissolved as a solute in a melt of a Group III element and a liquid phase epitaxial layer is grown from the solution. The Group III nitride crystal conventionally sold on the market consists, however, of fine powders produced by reacting a Ga oxide, such as Ga.sub.2 O.sub.3 in case of GaN, with ammonia. The surface of the GaN crystal is not easily wet due to the shape and there is consequently a problem that the powders are not easily dissolved in Ga.
Nitride crystal powders are expected to be applied as a luminescent material and a dopant material in liquid phase epitaxial growth of GaAs, GaP, and the like. The GaN crystal powders which are conventionally available on the market have a low impurity for the cost and are unsuitable as a starting material for vapor phase growth. Since the powders are not easily dissolved in a semiconductor solution due to the shape, there is a problem that the powders are not easily used as a material for liquid phase growth or a dopant material.
On the other hand, a method called hot pressing in which a substance as a material is charged into a cylinder and heated and molded while pressurizing it by a piston is generally used for molding ceramics and the like.
According to the hot pressing method, although a nitride such as AlN can be molded under pressure by using an appropriate binder, a single crystal growth has not been realized yet. This is because the melting point of a nitride crystal is very high and a nitride is decomposed before the temperature of the material reaches the melting point and nitrogen escapes from the material.